Background: Young men who have sex with men (YMSM) are at exceptionally high risk for HIV infection in the United States (US). Increasing awareness of HIV status and expanding access to Pre-Exposure Prophylaxis (PrEP) for high-risk HIV-uninfected YMSM is essential for reducing the population burden of disease; however, overall rates of testing and PrEP uptake among young MSM are suboptimal. Smartphone use has nearly reached saturation among youth in the US, making mobile technology a particularly promising tool for engaging youth, who are typically difficult to reach through standard clinic services. Overview: Informed by prior formative research with YMSM in the US, two distinct apps, LYNX and MyChoices, were developed to increase HIV testing and PrEP uptake among YMSM using different underlying theoretical models and user engagement approaches. In years 1-2 of the iTech, we will have optimized and pilot-tested these mobile apps for feasibility and acceptability. In the current project, we propose to: (Aim 1): Synthesize the results of two pilot studies conducted in Years 1-2 of the iTech of two HIV prevention apps (LYNX and MyChoices) among YMSM, in order to tailor and finalize the design of a RCT of app efficacy. Using a priori feasibility and acceptability criteria, we will determine whether both, only one or neither app will be tested in a full-scale RCT; (Aim 2) conduct an RCT to assess efficacy of LYNX and MyChoices to improve HIV testing and PrEP uptake among YMSM. If both apps are acceptable and feasible (Aim 1), 450 YMSM at four iTech SRVs will be enrolled and randomized to one of three conditions: a) MyChoices app, b) LYNX app, or c) control condition which consists of referrals to local HIV/STI testing resources. Participants will complete quarterly online assessments over 12 months of follow up. The primary outcomes will be proportion with 0 vs. ?1 HIV test during the 12-month interval (via self-report) and PrEP starts via self-report); (Aim 3): To develop a strategy to disseminate efficacious HIV prevention apps, to characterize cost metrics, and to provide recommendations that will allow implementing programs to fully consider the most appropriate ways to use technology-based solutions to increase HIV testing and PrEP uptake. Innovation: If either or both apps prove to be efficacious and cost-effective, this innovative technology has the potential to reach a large population of at-risk YMSM across the US, increase HIV testing and PrEP uptake among those at highest risk and reduce the burden of HIV in this population. Although there is expanding interest in the use of technology-based interventions to enhance HIV prevention efforts, few studies have rigorously evaluated the efficacy of these approaches. Further, no published study has compared two apps, developed with different theories of behavior change, head-to-head for efficacy in health behavior change.